Quote:
Originally Posted by Satchmo
oooh perhaps better in another thread then.
I guess I was referring to the speed of imaging extended stuff. So , with point sources like stars its the increased focal length that allows us to image fainter stars ( I assume that deepest magnitudes can be reached by increasing exposure time for a given aperture )
So given we want as fast a system as possible for extended objects, fainter limiting magnitude at a given exposure is reached via longer focal length gained by keeping F # constant and increasing aperture?
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Not quite, as far as I understand with point sources it's the aperture that defines the deepest magnitude and it is independent of focal length. For example my 14mm f/2.8 lens with 90mm front element gets fainter stars than 35mm f/1.4 lens with 55mm front element with the same camera and exposure length (although this is subjective - I have not done any measurements)
Somewhat outdated but still relevant info:
http://www.eskimo.com/~rachford/widefield/calc.html
[Edit: added another link]
http://starizona.com/acb/ccd/projectsfaint.aspx ("Capture the Deepest Amateur CCD Image" paragraph)
Alex